For a poster, the viewer’s eye moves from the top-left down towards the lower-center and back up to the right.
Place the most important piece of information first, in the upper left.
Then, place the next most important item in the lower center; possibly a nice chart with interesting data.
Along the upper-center section, you can place images, but not ones crucial to your message as this is an area not typically looked at initially.
Lastly, place your conclusions at the top right or bottom right.